March 28, 2011

Plate tectonics on Earth are heavily aided by the presence of water to weaken the integrity of the crust at subduction zones, and this lowered resistance helps oceanic plates sink below the lithosphere. Venus is approximately the same size as Earth, but it doesn’t have plate tectonics like Earth does. Some geophysicists like cite a myriad of variables that might determine whether or not an Earth-mass planet is active tectonically. Besides the presence of water, the thickness of the crust can determine the amount of resistance of a tectonic plate which will impact whether a plate will buckle or deform in the first place when it collides with another plate. A thicker crust is less likely to deform than a thinner crust. This is determined by how vigorous the internal convection of the mantle is, which is in turn determined by how hot the bulk of the planet is. A hotter interior causes convection of the molten material of the mantle to rise and sink faster, hitting the underlying material of the lithosphere harder and driving the movement of tectonic plates all the faster. Smaller objects like Mars or Mercury have a smaller mantle, which indicates a lack of a supply of fissionable elements in the core, and this means less radiogenic heat is produced from the breakdown of heavier elements. For these smaller planets, the mantle cools faster than an earth-sized planet and as the mantle hardens and cools, the crust gets thicker and tougher to subduct. So plate tectonics on a smaller planet shuts down earlier in that planet’s history, which leads to a whole host of problems for habitability. The tectonic cycle regulates (p.758) the amount of carbon dioxide and sulfur dioxide on the surface of the planet, so shutting down this system is disastrous. Carbon dioxide and sulfur dioxide build up, resulting in either an increase of these gases in the atmosphere or the crust.

Areios has a much hotter interior, which keeps the crust from forming too thick and pushes the tectonic plates so quickly that any crust generated from mid-ocean ridges would get recycled into the mantle quickly. This tectonic cycle is a fast-forward version of Earth’s tectonics, so carbon dioxide would get scrubbed from the atmosphere by hyperactive plate tectonics faster than volcanoes could spew it out. During a period of maximum glaciation, ice covers the continents, weighing down the plates and halting the process that scrubs carbon from the atmosphere. This builds up a greenhouse effect that eventually overcomes the frozen climate. It’s not until the end of a disastrous ‘Snowball’ period (p.759) that carbon dioxide levels rise to support planet-wide photosynthesis. More on that later…

The atmospheric composition of a planet is determined in part by the geology of the crust. As we’ve seen before, on Earth and Areios, carbon dioxide in the atmosphere gets pulled out of the air by minerals in the crust that get dragged back into the mantle by tectonic forces. But this isn’t the only way that the crust influences the composition of the atmosphere. When Areios was first forming, volcanoes would spew out water vapor and other volatiles that would eventually condense into the oceans. To be sure, just like on Earth, some of the water on the planet’s surface was delivered by ice-bearing comets (p. 752) and meteorites, but a significant amount of the planet’s early atmosphere was baked out of the crust. Like carbon dioxide, sulfur dioxide is released into the atmosphere by volcanoes, but because SO2 is so reactive, it doesn’t stay in the atmosphere like carbon dioxide does and is scrubbed out of the atmosphere quicker. This chemical exchange between the rocks and the air is only made more complex by adding the chemical reactions associated with life. Our instruments may one day be able to detect the presence of life based off their biosignatures it leaves in the atmosphere; the discovery of oxygen or methane in abundant quantities would all but confirm the existence of life there.

These interactions with the crust keep the composition of the atmosphere on Earth and Areios constant. However, in the history of both the Earth and Areios, a single prolonged episode in geologic history shot millions of tons of oxygen gas into the atmosphere. This may be the single most significant event in the history of life on these terrestrial planets because it means the rise of the complex organisms and eventually the humans and the Areia, but also it spelt the demise of any anaerobesthat couldn’t seek refuge in an anoxic environment. The Great Oxidation Event may have been a mass extinction for anaerobic life, but it leads to the creation of the most recognizable organisms on our planet; the Eukyotes.

March 22, 2011

On Earth, geologists can understand the geologic events of the past by analyzing the different rock types and using clues to discern what forces creating them. The rock cycle explains how the three different rock types come to be via natural processes of volcanic eruptions, weathering and erosion, and heat and pressure (and time). Igneous rocks are created when lava on the surface cools and hardens into a solid mass. The density, composition, and texture of igneous rocks can tell a geologist where and how it formed; granitic rocks (rocks that contain a higher proportion of silicate materials) tend to be less dense than basaltic rocks (rocks with a higher percentage iron, calcium, or magnesium metals and less silicates). A geologist can usually tell what an igneous rock is made out of by coloring at the color of the rock; darker means more basaltic, and lighter hues suggest a granite-type. And lastly, the texture of a rock tells a geologist how fast the rock cooled; rocks with a glassy texture cooled too quickly to form grains, which means the rock formed on the surface, where the different between the molten interior of the earth and the room temperature outside meant that rocks solidified rapidly. Large grains mean the rock had time to form slowly, which suggests that it formed under the Earth, when higher temperatures allowed mineral grains to slowly coalesce.

Igneous rocks are classified by their composition and their rate of cooling

What kind of rocks are the Earth’s crust made out of? The Earth’s crust is made up of silicates, minerals like quartz or olivine, and Areios shares the same basic bulk composition as Earth, with some minor chemical differences that we won’t need to worry about. The crust is made up of plates that are pushed by the convection of the underlying mantle at a snail’s pace; only a few centimeters a year. There are two different kinds of plates that vary by their age and composition. The oceanic plates overlie the Earth’s ocean andare made up of denser basaltic rocks and the continental plates which are less dense make up our continents and some of the underlying mantle. When these jostling plates come into contact with one another, they create volcanic eruptions, earthquakes, tsunamis and other natural hazards. Because oceanic plates are denser, when they collide with continental plates, they sink into the mantle and are partially melted and become part of the mantle. When two oceanic plates collide, the denser one will sink below the mantle. But when two continental plates collide, typically they ram up against one another to form mountain ranges like the Himalayan Plateau (a collision between the Indian and Asian plates). Plates can also move apart from one another in an event called rifting. Rifting occurs beneath the Atlantic Ocean along mid-ocean ridges where the material from the mantle rises and punches a hole through the crust, allowing magma to well up to the surface, where it hardens to form new oceanic crust. As more magma rises up it pushes older rockers farther out from the ridges, creating new crust material. Oceanic crusts will eventually get pushed into subduction zones where old crust gets recycled into the mantle. This system of continental driftdrives the movement of tectonic plates.

Areios has tectonic plates and despite the many differences in Terroan and Areiosan geology, the processes on both planets are analogous. Areios has roughly the same volume of land area covered by ocean as the Earth, but because Areios’ land area is larger, oceans make up a smaller percentage of the total land. This is important because water acts as a lubricant for subduction and this process that creates and destroys crust is responsible for the regulation of carbon dioxide. The biggest change in in the tectonic system from Earth to Areios is that the process on Areios happens much quicker, so it voraciously scrubs any carbon dioxide out of the atmosphere as quickly as it can be released from mid-ocean ridges. This makes it harder for photosynthesis to occur on Areios because CO2 is the fuel for photosynthesis. Along with the high levels of sulfur compounds that bleach chlorophyll and the lower levels of sunlight, photosynthesis doesn’t appear until later on in Areiosan history and the plant kingdom never arises on Areios at all. We’ll discuss how this omission impacts the biosphere later on, but for the time being it suffices to say that the animal kingdom on Areios would have to adapt to a world without green.

March 15, 2011

Areios is unique in this solar system in that it has liquid water on its surface, just like our Earth does. This liquid water helps to keep the climate moderate because of its high specific heat that allows water to absorb a tremendous amount of energy before it vaporizes. Water gets cycled throughout the environment through the perennial process of evaporation, condensation, and precipitation. Water also gets transformed chemically as it moves from the living to non-living environment. Water isn’t the only chemical to get recycled; nitrogen, phosphorus, sulfur, and carbon get converted into different forms through chemistry between the living and non-living parts of the environment. Areios undergoes more or less the same processes as the ones we find on Earth.

On Earth, the nitrogen cycle converts inert nitrogen gas into a form useable for life. Organisms called nitrogen fixers can break the triple bonds between the nitrogen atoms in N2 which few other organisms on Earth can do. These nitrifying bacteria appear mostly in the root of certain plants on earth and play a valuable role in supplying nitrogen to the environment, which is usually a limiting factor for plant growth. These bacteria can convert N2 to nitrate or nitrite, which plants can use. Other bacteria can convert nitrate or nitrite into ammonia. Before these organisms were around nitrate and nitrite were produced in lightning strikes, and production of usable nitrogen was limited. Still other bacteria called nitrifiers convert ammonia to nitrate while denitrifiers convert it back into nitrogen gas.

Phosphorus on Earth comes from rocks, which is released by weathering or through human intervention (think mining, extraction industries). When leached from rocks, phosphorus makes its way into the water and is quickly taken up by organisms because like nitrogen, phosphorus is a limiting factor for growth. When phosphorus gets into the water supply, it causes rapid algae growth to use up the available oxygen in the water, which causes the fish to suffocate in these dead zones at the mouths of major rivers. Right below phosphorus on the periodic table is arsenic; scientists recently discovered an organism that can substitute phosphorus in its DNA for arsenic. Because arsenic is so chemically similar to phosphorus, arsenic is deadly to humans because arsenic will replace any phosphorus in our bodies, but we can’t metabolize it. The arsenic cycle behaves analogously to the phosphorus cycle, which is what we would find on Areios instead of the phosphorus cycle. Instead of a life form that can tolerate higher levels of arsenic like GFAJ-1, life on Areios all but exclusively uses arsenic wherever phosphorus is used in biochemistry on Earth; in ATP, nucleic acids, and in the minerals that make up their bones and teeth. This alternative biochemistry makes the Areia fundamentally different from Terroa, or what I what classify as life from Earth, and this biochemical disparity effectively segregates these two trees of life because we Earthlings are toxic to the Areos as much as they are toxic to us. Contact with arsenic-life would be disastrous for both parties because of how poisonous we would be to one another. Perhaps even touching one another might be hazardous!

GFAJ-1 may be the first organism discovered that can replace the phosphorus in its DNA for a compound called arsenate.

The sulfur cycle is the most radically different from our world to Areios because sulfur plays a much bigger role in Areiosan metabolism. We’ll talk more about the role sulfur and sulfate-reducing bacteria in upcoming posts, but for now, understand that sulfur compounds are important in Areiosan ecosystems in a way that is significantly different from how it’s used on Earth. On Earth, sulfur is produced in hydrothermal vents in the form of hydrogen sulfide and in volcanoes as sulfur dioxide. Plants release minute traces of carbon vinyl sulfide in respiration and burning coal produces sulfur trioxide. These gases come in contact with water vapor and a series of reactions creates trace amounts of sulfuric acid droplets. This acid rain falls to the earth and reacts with the crust to form sulfide or sulfate minerals. These minerals eventually become part of the crust and get subducted into the mantle where sulfur gets spewed out by volcanoes and hydrothermal vents once more. Human presence alters this cycle through mining and burning coal, which has exacerbated acid rain damage, particularly in the Northeastern U.S. and Eastern Europe.

On Areios, the sulfur cycle is a bit more complex than on Earth. For one thing, some organisms build yellow mounds out of crystalline sulfur, like coral, or termite and ant hills on Earth. Some creatures incorporate a sulfur-eating bacterium on their skin that produces sulfuric acid to ward off predators. Areiosan cell metabolism relies more on sulfur compounds than our Terroan life. The earliest eukaryotes on Earth were thought to resemble an archean cell housing a rickettsia-esque bacteria (like the one that causes Rocky Mountain spotted fever). We’ll go more in depth on this endosymbiotic arrangement later, but it was thought that the rickettsia could detoxify peroxides in the cell, converting it to water while eliminating any oxygen that would be deadly to an anaerobe. On Areios, the first eukaryotic cells were meant to detoxify hydrogen sulfide and convert it to water and a solid sulfur precipitate.

The carbon cycle is the final biogeochemical cycle we’ll discuss and is probably the one most people are familiar with; with climate change such a hot issue right now for the political establishment, people have begun to pay more attention to the processes that release or sequester carbon in the environment. Carbon dioxide is a stable gas in the atmosphere that traps heat and gives both Earth and Areios a comfortable greenhouse effect that would otherwise freeze the planet. Our current climate predicament has less to do with the amount of carbon dioxide in the atmosphere (because carbon dioxide levels have fluctuated wildly over the last four billion years), but it’s dire because of the rapidity that the changes have been taking place and because human activities influence carbon dioxide levels to an unprecedented extent. The greenhouse gas methane is 40 times more powerful than CO2 and on Areios as well as Earth; this gas plays a much more powerful role in the climate. We’ll see why this is important in an upcoming post…

March 8, 2011

The Earth’s crust is made mostly out of oxygen and silicon, but that need not be the case for terrestrial planets. Terrestrial planets can be iron-rich, carbon-rich, water-rich, or silicate-rich. As terrestrial Earth-type planets go, any planet with a significant amount of mass will accumulate an atmosphere, but if the planet gets too massive, it will take on too much atmosphere and become a gas giant more akin to Neptune or Uranus. If a planet is too small, it won’t accumulate much of an atmosphere at all and that will prevent liquid water from accumulating on the surface making the surface of the planet dry and frozen like Mars. A smaller planet will have its liquid outer core and mantle solidify faster, so volcanism and the planet’s magnetic field will shut down much quicker than on Earth. With no volcanism to replenish the atmosphere, no magnetic field to keep solar wind at bay, and a generally smaller gravitational field that can’t hold on to as much atmosphere, smaller planets are less habitable than Earth-mass planets and greater and aren’t habitable for as long, either.

A terrestrial planet more massive than Earth but less than about 10 times the mass of the Earth is considered a super earth. Anything more than 13 times the mass of the Earth would cause the planet’s gravity to hold on to too much gas and the thick envelope of a gas giant’s atmosphere would form. One astronomer suggested that a gas giant could be stripped of its atmosphere and may became a chthonian planetif a nearby massive star goes nova and tears the atmosphere off the planet, which you would expect to find in a galactic area with high-metallicity and many nearby aging stars. Super earths can be classified by their composition and internal structure and they come in two major varieties; water-rich and rocky super earths.

Iron-rich planets would form closer to the protoplanetary disk of the star they orbit, where metal content is highest. Planets rich in iron would cool quicker than silicate-based planets and that means volcanism, plate tectonics and a magnetic field would halt much sooner on a planet that cools that quickly. Mercury in our solar system is most similar to this; Mercury’s lighter silicate crust could have been boiled away, leaving behind the iron core, which makes up a greater proportion of the planet’s mass.

Our Sun has a carbon: oxygen ratio of about 0.5, so CO2 is common in the atmosphere of planets with silicate crusts. Bur for super earths that would accumulate much more carbon than a planet like Earth, there would be less CO2 in the atmosphere and the crust would be made predominantly of silicon carbide and graphite, and a layer of diamonds would be present deeper within the crust as graphite gets squeezed by heat and pressure to form diamonds. During volcanic eruptions, molten diamonds would gush from the volcano along with silicon carbide.

Planets covered by ocean are called water worlds, and because of the pressure of the atmosphere, this water would form a layer of ice VII over the entire surface of the planet. Ice VII is a truly alien form of water that would be crushed into a solid form at near-boiling temperatures. Water worlds resemble planets like Uranus or Neptune that would have migrated closer to their star and melted. These planets would be composed of a volatile content identical to the ice-bearing comets where their water would have come from. Rocky-type super earths might have the amount of water comparable to what one might find on Earth, but because the planet has a much bigger radius, oceans would straddle less of the planet’s surface, like it does on Areios. In fact, the amount of volatile content like water that gets captured by a planet might vary on an order of magnitude of about 1,000. This means that a planet could wind up with next to no water on its surface, or it might be flooded with water all over its surface. While a water world may be habitable to life, space faring intelligent life can’t arise on a water world because if a species can’t even build fire, these creatures certainly couldn’t discover rocketry, radio telescopes or even metallurgy. This means that unless we build a rocket and fly to one of these water worlds, we may never come in contact with an intelligence that dwells there.

A silicate-rich planet would resemble the terrestrial planets in our solar system; the crust would be made of silicon dioxide mostly, and plate tectonics would control the amount of carbon dioxide in the atmosphere by virtue of subduction. Areios is a silicate-rich planet like our Earth, but because of its more massive size, volcanism wipes the atmosphere clean of carbon dioxide just as fast as volcanoes can spurt it out. The same volcanic processes on Earth appear on Areios, but at a much faster pace. The crust on Areios is the same thickness as on earth, yet with a larger mantle and more gravity pushing down on the crust; plate tectonics operate in the same mechanism as they would on Earth, with the denser basalt plates getting driven beneath the lighter continental crust.

March 5, 2011

Richard Hoover of NASA’s Marshall Space Flight Center reported in the March Issue of the Journey of Cosmology that he had discovered evidence of microfossils in carbonaceous chrondites that fell to the Earth. Hoover’s research suggests that these fossils are not Earthly contamination, but evidence of life that lived on another body in the solar system. Fragments of their original environment traveled through space until these most primitive meteorites arrived to the Earth via meteorite impact. Here is a link to Hoover’s recently published article “Fossils of Cyanobacteria in CI1 Carbaceous Meteorites: Implications to Life On Comets, Europa, and Enceladus”. As reported by the Journal of Cosmology, “Members of the scientific community were invited to analyze the results and to write critical commentaries or to speculate about the implications. These comments will be posted on March 7 through March 10 2011.”

Expect more posts next week on any major developments surrounding Dr. Hoover’s recent discovery.

March 1, 2011

In the very center of Areios is the core, a dense ball of iron and nickel that sloshes around inside the planet, generating a magnetic field like the one on Earth. The core is divided into an inner and outer layer, based on density and these two layers spin at different rates, causing a magnetic field to form from an induced dipole moment. The magnetic field on Earth is generated by the molten iron and nickel that gets swirled around by the tug of the Earth’s orbit. The magnetic field was actually induced by the magnetic field generated from the Sun, and kept going by the motion of the liquid iron outer core which can conduct electricity as it was churned by the Coriolis Effect.Magnetic Pole reversal

Because Areios will take longer to cool, its core won’t differentiate into inner and outer layers until much later in time relative to how it happened on Earth. Because the core won’t differentiate at first, there won’t be a magnetic field on Areios until the planet’s insides settle down. This is important because that magnetic field keeps solar wind from stripping the atmosphere away and it keeps out deadly radiation that would attack the organic machinery of cells. In the book The Life and Death of Planet Earth, Peter Ward describes what some astrobiologists believe happened to Venus and Mars when the magnetic field of a planet stops; solar wind tears water into hydrogen and oxygen, boiling away the atmosphere until the atmospheric pressure prevents water from collecting on the surface at any temperature. The result: a dry and frozen world like Mars, or a dry and broiling world like Venus. Because Areios won’t develop a magnetic field until later on, life probably couldn’t start until the radiation bombarding the planet could be deflected. Thankfully, Areios regenerates its atmosphere through volcanic venting and it has enough gravity to hold on to some of the gases that would otherwise leak out of an Earth’s sized planet’s atmosphere, so the atmospheric stripping one would expect from Hemera’s solar wind can be kept at bay, or at least mitigated for a while.

This magnetic field reverses from time to time, and we have evidence of this on earth in iron-bearing minerals that have spewed out onto the crust from the mantle. In the Atlantic Ocean, there are areas where new crust is being created; magma from the mantle forces its way onto the surface as lava that cools and forms the ocean floor. As it solidifies, new material pushes the old material out of the way as more lava wells up from the mantle in a process called seafloor spreading. Magnetized iron in mineral crystals from the mantle record which way the magnetic field is spinning at the time when it hardens into rock. These rocks record a trend of increasing or diminishing magnetization of iron in the mantle and they show evidence that over geologic time, the poles will reverse with the North Pole flipping down to the South Pole and vice versa. During the process where the magnetization flips, there are periods of weak magnetization that can be disastrous for life because this causes more ionizing radiation to leak through the atmosphere.

On Areios, the thicker mantle keeps the insides of the planet too hot to differentiate the mantle and core into two distinct layers until later on in Areios’ history. That means that for the earliest period in Hemera’s stellar life cycle, Areios is unprotected by the cosmic rays that Hemera would bring onto Areios’ surface. Only after Hemera stops blasting the surface of Areios with radiation does Areios develop a magnetic field. Four billion years after the formation of Areios, we see a number of habitability factors line up for the first time; Hemera stops having such violent solar flares, the bulk structure of the planet settles down to trigger its magnetic field, the planet’s volcanism shoot out less gas, which causes a geologic ice age period. All of these converging factors lead to the first Areiosan lifeforms, the Areia.